This award supports theoretical research and education on heavy electron materials and local moment physics. The new concepts required to understand these low-energy materials, will, in many cases scale up in energy and temperature to describe related phenomenon in transition Metal materials, such as copper and iron-based superconductors. Many of the key questions in this area touch upon issues of fundamental importance, such as the physics of quantum criticality, mechanisms of anisotropic superconductivity, and the origins of non-Fermi liquid behavior. Motivated by the discovery of two new and unusual heavy fermion superconductors, the PI will apply a new type of large-N expansion for heavy electron superconductivity that he has recently developed. Fresh insights in heavy electron quantum criticality and the discovery of a Kondo spin liquid material inspire the PI to embark on a new set of investigations into the phase diagram of the Kondo lattice. A magnetically tuned critical end point in a multiferroic oxide has recently been discovered. The PI will pursue a new research program to study field-tuned quantum criticality in multiferroic materials.
Research into the various aspects of "hard condensed matter physics" plays an essential driving role in both the development of new physics concepts, and new ideas for materials of the future. For example, the remarkable tendency of quantum critical points to nucleate superconductivity and other new phases of matter is of great interest to materials development; on the other hand, the new universality classes of quantum phase transition are of great fundamental interest and, like their classical predecessors in statistical mechanics, may enjoy generalization and future application in the realm of cosmology and particle physics. This is wonderful area for students to learn the advanced methods of theoretical physics, while maintaining an intimate contact with experimental physics and real materials.
The PI is strongly committed to diversity in physics, with a long record of support for women in physics. Two women graduate students are currently working in his group.
NON-TECHNICAL SUMMARY This award supports theoretical research and education on a class of materials known as heavy electron materials. The interplay of electrons in itinerant quantum states and quantum states localized around a rare-earth or actinide atom leads to an unusual metallic state with properties that are consistent with the textbooks. The properties of these materials are further complicated by the existence of quantum phase transitions - phase transitions that occur at the absolute zero of temperature but with a powerful influence that can be felt over a range of temperature. The PI will build on and use a promising new method that he has developed to gain insight into two recently discovered materials in this class that are superconductors and into heavy electron materials more generally. The PI will also study the superconducting states of these materials. The PI will further explore the role quantum phase transitions play in newly discovered multiferroic materials which are a promising class of materials for future electronic devices.
The study of the superconducting states and the unusual metallic states which give way to superconductivity contributes to the intellectual foundations of materials research and superconductivity. This knowledge may lead to harnessing the ability of superconductors to carry electric current with no to very low power dissipation in a practical way leading to significant energy savings.
The PI is strongly committed to diversity in physics, with a long record of support for women in physics. Two women graduate students are currently working in his group.
" has focussed on the physics of strongly correlated electron materials, with an emphasis physics of heavy fermion systems - materials where the f-component of the electron fluid is heavily localized. We are particularly interested in the behavior of these metals when tuned to the brink of magnetic instability - for it is here that the unusual metallic behavior, and the predisposition to superconductivity are most prevalent. 1. Developing a new theoretical understanding of heavy fermion superconductivity and its interplay with magnetism. Of particular interest - the question of how local magnetic moments participate actively in the heavy fermion condensate. To this end we have developed a new class of theory - involving the co-operative pairing of magnetic moments and electron pairs to form "composite pairs". These objects are bosons, and it is the condensation of these composite pairs that we conjecture is primarily responsible for the development of superconductivity in a class of heavy fermion superconductors called "115 materials". 2. Developing our theoretical understanding of quantum criticality, by properly identifying the global phase diagram and seeking experimental input that can help guide the theory. To this end, we have worked with two experimental groups in Japan and Europe to understand the emergence of a new non-Fermi liquid phase between the antiferromagnet and heavy fermi liquid. We have worked in close conjunction with experimentalists in Austria and Japan to understand two non-Fermi liquid phases of this sort: Ge doped YbRh_2Si_2 and YbAlB_4. In the latter compound, we developed a scaling treatment of the magnetization that showed that the material is intrinsically quantum critical without any external tuning parameter. This work was published in the journal Science. 3. Developing a theory for the hidden order that develops in the heavy fermion superconductor URu2Si2. Motivated by the discovery that the quasiparticles in the hidden order phase of this system have a perfect Ising anisotropy, we have shown that in this system, the Ising order derives from hybridization with an integer spin, 5f^2 doublet staet - a 'non-Kramers doublet'. The development of a hybridization that mixes tan integer spin 5f^2 state with the half-integer spin conduction electrons requires breaking time-reversal in a novel way. A hybridization between a half-integer and integer spin state requires an order parameter that itself carries half-integer spin, an order parameter which changes sign under double time-reversal. Such a state breaks time-reversal symmetry, but it also breaks double time-reversal symmetry, permitting the mixing of integer and half-integer spin state. A key paper describing our discovery was published in the journal Nature.
